In applications requiring high intensity laser-plasma interactions, it is often desirable to maintain high optical intensity over long interaction distances. Conventional optical components such as mirrors and lenses cannot operate at laser intensities above the damage threshold for the materials forming these components. As a result, such optical elements must be placed far from the laser focus, limiting the interaction distance of the focused pulse to the Rayleigh range.
This limitation can be overcome by focusing the laser beam into a plasma channel consisting of a preformed plasma having a minimum density in the center, for example, a plasma having a parabolic radial density profile. The plasma channel acts as a waveguide for the laser pulse combating diffraction and maintaining the pulse intensity over an extended distance. See C. G. Durfee and H. M. Milchberg, “Light Pipe for High Intensity Laser Pulses,” Phys. Rev. Lett. 71, 2409 (1993) and D. Kaganovich et al., “High efficiency guiding of terawatt subpicosecond laser pulses in a capillary discharge plasma channel”, Phys. Rev. E, 59, R4769, (1999) (“Kaganovich 1999”); see also T. R. Clark and H. M. Milchberg, “Time- and Space-Resolved Density Evolution of the Plasma Waveguide,” Phys. Rev. Lett. 78, 2373 (1997); and A. Butler, D. J. Spence, and S. M. Hooker, Guiding of High-Intensity Laser Pulses with a Hydrogen-Filled Capillary Discharge Waveguide,” Phys. Rev. Lett. 89, 185003 (2002).
Plasma-channel guiding of ultrashort laser pulses is a key component for laser-based particle acceleration techniques such as laser wakefield acceleration (LWFA). See G. M. Mourou, T. Tajima, and S. Bulanov, “Optics in the relativistic regime,” Rev. Mod. Phys. 78, 309 (2006). LWFA can produce high-quality, low-emittance, ultrashort bunches of mono-energetic electrons. See V. Malka, “Laser plasma accelerators,” Phys. Plasmas 19, 055501 (2012). However, several significant technical challenges still separate LWFA from conventional radio-frequency (RF) accelerators in particular, maintaining the driving laser pulse intensity over a long (>10 cm) distance in a low (<1019 cm−3) plasma density.
Plasma channel guiding of laser pulses has been demonstrated experimentally using channels created by one of two techniques. The such technique uses solid wall structures based on capillary discharges. See A. Butler, D. J. Spence, and S. M. Hooker, “Guiding of High-Intensity Laser Pulses with a Hydrogen-Filled Capillary Discharge Waveguide,” Phys. Rev. Lett. 89, 185003 (2002); see also Kaganovich 1999, supra. The second technique uses wall-free channels based on axicon-focused lasers. See Durfee, supra, and Clark, supra. In both cases, the waveguide is initiated by the on-axis heating of a uniform cold plasma column or neutral gas. Hot gas near the axis expands radially, forming a hollow density channel suitable for guiding.
The capillary discharge technique for creation of a plasma guiding channel uses a dielectric tube several hundred microns in diameter. See Y. Ehrlich, A. Zigler, C. Cohen, J. Krall, and P. Sprangle, “Guiding of High Intensity Laser Pulses in Straight and Curved Plasma Channel Experiments,” Phys. Rev. Lett. 77, 4186 (1996). The capillary can be back-filled with gas (see Butler, supra) or filled with wall material ablated when a high voltage breakdown launches from a pair of electrodes located at each end. See D. Kaganovich, P. Sasorov, Y. Ehrlich, C. Cohen, and A. Zigler, “Investigations of double capillary discharge scheme for production of wave guide in plasma,” Appl. Phys. Lett. 71, 2925 (1997) (“Kaganovich 1997”). This produces collisional heating near the axis while the region near the wall stays relatively cold, setting up conditions for hollow plasma channel formation.
The wall-free techniques employ a high energy, long laser pulse to ionize and heat a plasma column produced from either clustered (see A. J. Goers, S. J. Yoon, J. A. Elle, G. A. Hine, and H. M. Milchberg, “Laser wakefield acceleration of electrons with ionization injection in a pure N5+ plasma waveguide,” Applied Physics Letters 104, 214105 (2014)), or un-clustered (see Geddes, supra) gas jets. In order to produce an axially extended channel, the hydrodynamic heater pulse must be line-focused into the gas column by either an axicon (conical lens), see Durfee, supra, or a cylindrical focusing optic, see Geddes, supra.
Creation of long channels requires high laser energy and precise co-linear alignment, making this scheme more difficult to implement than capillary channels. For shorter distances (one centimeter or less), a self-guided laser in clustered gas can be used to initiate a shock wave-based guiding channel. See V. Kumarappan, K. Y. Kim, and H. M. Milchberg, “Guiding of Intense Laser Pulses in Plasma Waveguides Produced from Efficient, Femtosecond End-Pumped Heating of Clustered Gases, Phys. Rev. Lett. 94, 205004 (2005).
The current world record for LWFA electron energy, 4.2 GeV, was demonstrated using a 9-cm long capillary discharged waveguide. See W. P. Leemans, A. J. Gonsalves, H.-S. Mao, K. Nakamura, C. Benedetti, C. B. Schroeder, Cs. Toth, J. Daniels, D. E. Mittelberger, S. S. Bulanov, J.-L. Vay, C. G. R. Geddes, and E. Esarey, “Multi-GeV Electron Beams from Capillary-Discharge-Guided Subpetawatt Laser Pulses in the Self-Trapping Regime,” Phys. Rev. Lett. 113, 245002 (2014). While in principle, a discharge capillary could be extended beyond 10 cm, neither effective guiding nor acceleration has been demonstrated at such lengths. It appears that the limitation is discharge formation, but this remains poorly understood due to difficulties in diagnosing the plasma within a capillary. Standard diagnostic techniques, such as optical interferometry, cannot be used to transversely probe the plasma within the capillary. This also makes it difficult to monitor the performance of the waveguide. Additionally, the dielectric wall is subject to damage by the laser field, discharge current, and plasma.